1. Field of the Invention
The invention relates generally to television circuitry. More particularly, the invention relates to signal processing circuitry for the audio portion of a television signal.
2. Description of the Prior Art
Television audio signals have a dynamic range of greater than 20 dB. Excursions to 100% modulation (0 dB) produce higher sound levels, which are annoying to television viewers. Signal conditioning, by equally compressing the audio signal, is sometimes done on the audio portion of the transmitted television signal. The compressed signal is then raised to a higher level of audio modulation at the television receiver, resulting in an increased sound level during portions of a program, such as a commercial.
Prior art patents and other publications show the use of Zener diodes in a sound-limiting circuit for a pair of headphones. In L. Persson (U.S. Pat. No. 4,224,470), the Zener diodes are in a shunt relationship with the earphone speakers to limit voltage, while a field effect transistor (FET) limits current. Voltage limitation is suggested in Persson through a variable resistor connected in series with the earphone speakers.
D. Janssen (U.S. Pat. No. 4,079,211) shows a protective circuit for a telephone receiver. A shunt portion of the circuit includes a junction transistor combined with a Zener diode and a capacitor. A voltage divider is included in the shunt circuit. The circuit also includes a parallel bridge circuit and series variable resistor. The purpose of the Janssen circuit is to protect integrated circuits of a telephone set from undesired signal conditions, such as incorrect polarity and short pulses of high voltage. A control signal turns on a clamping circuit when a given threshold voltage is exceeded.
In a publication, Automatic Electric Technical Journal, Vol. 3, April 11, 1962, pp. 55-62, a discussion is made of a device operative on the variations in the voice applied to establish a constant level of output in spite of variations. A transformer is used for impedance matching.
German Pat. No. 2,756,300 shows loudspeaker adjustments for setting the maximum permissible power to the loudspeaker. A series resistor is selected in accordance with the impedance of the amplifier. The maximum power is set by an adjustable or variable resistor. This is a volume adjustment only, no compression of the audio signal at a set level occurs.
Other simple circuits for controlling the signal to an output or speaker, including a series resistor and parallel resistor, are seen in R. Rockwell (U.S. Pat. No. 2,171,048). D. Williams (U.S. Pat. No. 4,064,362) shows a CMOS with a FET employed to automatically limit voltage and the sound produced by a speaker in response to a signal. The Williams patent is directed to earmuffs which would allow only certain audio signal ranges to be received by the human ear.
The prior art tends to clip complex wave forms, thereby changing the quality of sound produced. Some of the prior art limits current only with respect to a single polarity, rather than both positive and negative signals, as is accomplished by the present invention. Rather than clipping the wave form abruptly at a level, the present invention actually reduces the voltage, acting as a resistance, to compress the audio signal.
A separate power supply for the circuit is not required, as is typical in the other prior art described, which have the ability to compress. Those circuits in the prior art which do not utilize power supplies, such as Persson, clip the signal, distorting the audio. If a power supply is used, the circuit is more expensive.